docs: def _get_doc():
      '''Get a description of all the known available features'''
      def get_docstring(func):
          '''extract doctstring, if possible'''
          docstring = ':\n'
          if func.__doc__:
              docstring += _indent(func.__doc__, 2)
          return docstring
  
      ret = ['\nNeurite features (neurite, neuron, neuron population):']
      ret.extend(_INDENT + '- ' + feature + get_docstring(func)
                 for feature, func in sorted(NEURITEFEATURES.items()))
  
      ret.append('\nNeuron features (neuron, neuron population):')
      ret.extend(_INDENT + '- ' + feature + get_docstring(func)
                 for feature, func in sorted(NEURONFEATURES.items()))
  
      return '\n'.join(ret)